Natural resources such as gas, oil, and water residing in a subterranean formation can be recovered using well-known techniques. The steps taken to prepare for the recovery of such resources usually require the use of various fluids. For example, drilling fluids or muds are typically circulated through well bores as they are drilled into the formation. During the drilling process, the drill bit generates drill cuttings that consist of small pieces of shale and rock. The drilling fluid carries the drill cuttings in a return flow stream back to the well drilling platform. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run in the well bore. The drilling fluid is then usually circulated downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the well bore.
Another fluid known as a gravel packing fluid having a relatively large grained sand, i.e., gravel, suspended therein also may be utilized to prevent migration of smaller grained sand from the subterranean formation into the well bore and to maintain the integrity of the formation. In particular, a permeable screen may be placed against the face of the subterranean formation, followed by pumping the gravel packing fluid into the annulus of the well bore such that gravel becomes packed against the exterior of the screen. In addition, a cement slurry may be pumped into the well bore during a primary cementing process in which the cement slurry is placed in the annulus of the well bore and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the well bore and seal the annulus. Subsequent secondary cementing operations, e.g., completion and work over operations, may also be performed using cement slurries.
Yet another fluid, i.e., a fracturing fluid, is typically used to fracture the subterranean formation. The fracturing fluid is pumped into the well bore at a rate and a pressure sufficient to form fractures that extend into the subterranean formation, providing additional pathways through which fluids being produced can flow into the well bores. The fracturing fluid is usually a water-based fluid containing a gelling agent, i.e., a polymeric material that absorbs water and forms a gel as it undergoes hydration. The gelling agent serves to increase the viscosity of the fracturing fluid. The fracturing fluid also typically includes particulate matter known as a proppant, e.g., graded sand, bauxite, or resin coated sand, may be suspended in the fracturing fluid. The proppant becomes deposited into the fractures and thus holds the fractures open after the pressure exerted on the fracturing fluid has been released.
The viscosities of conventional fluids used in a well bore usually decrease with increasing temperatures. As such, the fluids undesirably undergo thermal thinning as they pass down the well bore where they are exposed to increasing temperatures. Viscosification agents such as gelling agents may be added to the fluids to increase their viscosities. However, the resulting fluids are very viscous at the earth's surface and thus require relatively high pump pressures to be conveyed downhole. In addition, they also experience thermal thinning as they pass down the well bore.
Unfortunately, the thermal thinning of fluids as they pass down the well bore typically leads to various problems, depending on the type of fluid involved. For example, the drilling fluid may be unable to suspend drill cuttings therein as it flows back to the surface. Thus, the drill cuttings may settle out of the drilling fluid and become deposited in undesired locations in the well bore. Furthermore, those fluids containing particles such as the cement slurry, the gravel packing fluid, and the fracturing fluid may experience settling of the particles as the fluids are pumped down the well bore. As a result, the particles are not transported to their proper locations in the well bore. Further, in the absence of such particles, the density of the fluids may drop to a level at which they are incapable of withstanding relatively high fluid pressures downhole, particularly in the case of a high density cement slurry. A need therefore exists for maintaining the viscosity of fluids as they are passed into a well bore so as to prevent the settling of materials in those fluids.